An electrode for use in a polymer electrolyte fuel cell is sandwiched between two separators in a polymer electrolyte fuel cell. Such an electrode is configured to have, on each side of a polymer electrolyte membrane, a catalyst layer formed on the surface of the polymer electrolyte membrane and a gas diffusion layer formed on the outer side of the catalyst layer. As an individual component for forming a gas diffusion layer of an electrode, gas diffusion electrodes have been distributed. As the required performance of such a gas diffusion electrode, for example, gas diffusivity, electrical conductivity for collecting the electricity generated in the catalyst layer, water drainage for efficiently removing moisture generated on the catalyst layer surface, and the like can be mentioned. In order to obtain such a gas diffusion electrode, generally, an electrically conductive porous substrate having both gas diffusion capability and electrical conductivity is used.
As an electrically conductive porous substrate, specifically, a carbon felt, a carbon paper, a carbon cloth, or the like made of carbon fiber is used. In particular, in terms of mechanical characteristics and the like, carbon papers are believed to be the most preferable.
In addition, a fuel cell is a system that electrically extracts energy generated from a reaction between hydrogen and oxygen to produce water. Therefore, when the electrical load increases, that is, when the current to be extracted outside the cell is increased, a large amount of water (water vapor) is produced. At a low temperature, such water vapor is condensed into water drops, blocking pores of the gas diffusion electrode. As a result, the amount of gas (oxygen or hydrogen) supplied to the catalyst layer decreases, and when all the pores are blocked eventually, power generation ceases (this phenomenon is called “flooding”).
In order to minimize the occurrence of flooding, in other words, in order to maximize the current value that causes flooding, a gas diffusion electrode is required to have water drainage. As a means for enhancing the water drainage, usually, a gas diffusion electrode including an electrically conductive porous substrate that has been subjected to a water-repellent treatment is used (see Patent Documents 1, 2, and 3). With respect to the water-repellent treatment, according to a common technique, the above electrically conductive porous substrate is immersed in a dispersion prepared by dispersing a water repellent in water or an organic solvent (see Patent Documents 1, 2, and 3).
In addition, when the water-repellent-treated electrically conductive porous substrate as described above is directly used as a gas diffusion electrode, because its fiber mesh is coarse, large-size water drops are produced as a result of the condensation of water vapor. Thus, this is insufficient for completely suppressing flooding. Therefore, in some cases, on a water-repellent-treated electrically conductive porous substrate, a coating liquid having dispersed therein electrically conductive microparticles, such as carbon black, is applied, then dried, and sintered, thereby providing a so-called microporous layer (see Patent Documents 1, 2, and 3).
For fuel cell vehicle applications, the output of high power is required in the driving modes including starting, high-speed operation, hill-climbing, etc., and thus high output is required. In addition, in order to achieve high output, it is necessary that oxygen or hydrogen from the gas channel of a separator inside the fuel cell passes through the gas diffusion layer and quickly diffuses into the catalyst layer.
For this reason, a gas diffusion electrode is required to have high-level gas diffusivity. Considering such requirements, it is preferable that an electrically conductive porous substrate for a gas diffusion electrode is as thin and porous as possible so as to facilitate the diffusion of gas. When a microporous layer is applied under such circumstances, a coating liquid for forming a microporous layer (so-called microporous layer coating liquid) penetrates into the substrate having a small thickness and a high porosity. In an extreme case, the coating liquid bleeds through to the backside of the substrate, whereby the production process is contaminated with the coating liquid. Therefore, as a result of cleaning, for example, the productivity decreases. In addition, when the microporous layer coating liquid penetrates into the substrate, pores inside the substrate are blocked, making gas diffusion difficult, whereby the power generation performance may decrease.
In order to suppress the penetration of the microporous layer coating liquid into the electrically conductive porous substrate, Patent Document 3 discloses a technique in which the substrate after a water-repellent treatment is sintered to decompose and remove the surfactant in the water repellent, thereby preventing the penetration of a microporous layer to be applied later. However, according to the method disclosed in Patent Document 3, the water-repellent-treated substrate and the microporous layer do not stick well to each other. Thus, there has been a possibility that a part of the microporous layer comes off during the assembling of a fuel cell, whereby the microporous layer cannot achieve its original role.